We might have heard these words SI and CI engines when buying a brand new vehicle. But do you know what these words mean and what is the significance of them? SI engine or Spark Ignition engine and CI or Compression Ignition engine are the two most common classifications of an internal combustion engine. But how can we tell the difference between the two types?
In spark ignition engine, combustion takes place due to the spark produced by a spark plug inside the combustion chamber. Here combustion is a constant volume process. On the other hand, in compression ignition, combustion takes place due to the heat of compression. There is no spark plug present; instead there is a fuel injector. And combustion is a constant pressure process.
There are a few differentiating parameters used to compare the two engine types are basic cycle, type of fuel, injection of fuel, ignition process, compression ratio, and thermal efficiency. We will start by taking a look at each of those mentioned parameters.
SI engine works on the principle of the Otto cycle or constant volume heat addition cycle. The fuel used in SI engines is always petrol as it is highly volatile fuel and has a high self-ignition temperature; hence it needs a spark plug to ignite. In order to make the fuel flammable, a mixture of petrol and air is introduced inside the combustion chamber. To mix the air and fuel, a carburetor is needed. The compression ratio of the SI engine is limited to the range of 6:1 to 10:1. This is because, at higher compression ratios, the engine starts to knock, which can ultimately damage the SI engine. The compression ratio is also determined by the antiknock quality of the fuel used. Due to the limitation in the compression ratio, the maximum thermal efficiency achieved is lower.
Alternatively, the CI engine works on the Diesel cycle, or the constant pressure heat addition process. As the name suggests, the CI engine uses diesel as the working fluid, due to its low self-igniting temperature. It also has a non-volatile nature. In place of the spark plug, the CI engine has a fuel injector, which injects high-pressure fuel directly into the combustion chamber. A fuel pump is also needed to pressurize the fuel. The self-ignition occurs due to the high temperature of air because of the compression process. Here, a spark plug and an ignition system are not required. The compression ratio in CI engines achieved can range from 16:1 to 20:1. These engines are constructed to withstand the knocking, and the upper limit is based on the weight of the engine. The higher the compression ratio, the higher the weight will be and a stronger construction will be provided. Due to such high compression ratios, much higher maximum thermal efficiency and be achieved.
Authored By:- Cdt. Shubham Shivne, TMI
